In recent years, the development of liquid crystal display elements is remarkable, and these display elements are mostly directed to the application of nematic liquid crystals. On the other hand, liquid crystal displays of different driving modes, utilizing characteristics of other liquid crystal phases have also been intensively researched for practical use, and as one of these devices, there is a thermal writing projection device utilizing a thermooptical effect of a smectic A phase. The mode of such a device can provide a display of a far larger area and a far higher capacity in the number of figures, as compared with a conventional TN mode (twisted nematic mode).
Characteristics required for the liquid crystal materials used in this field include the following:
(1) a wide smectic A phase range (preferably, a smectic A phase range down to 0.degree. to -20.degree. C.),
(2) an extremely narrow nematic phase range,
(3) a high birefringence index,
(4) a small specific heat and a low phase transition energy; and
(5) a large dielectric anisotropy value.
Of these requirements, the items (1), (2) and (3) are considered to be important for improving the quality level of display such as contrast on a projected screen. As to liquid crystal materials having the above-mentioned characteristics, no single compound has yet been found, and hence mixtures of various liquid crystal compounds have now been investigated. Examples of these liquid crystal compounds can be represented by the following general formulae: ##STR5##
Furthermore, examples of compositions of these compounds are set forth in Table 1.
TABLE 1 ______________________________________ Compo- mp SN NI NR sition Compounds (.degree.C.) (.degree.C.) (.degree.C.) (.degree.C.) ______________________________________ a I.sub.8, I.sub.10, I.sub.15 24 73.5 74 0.5 b II.sub.8, II.sub.10 12 37 42 5 c II.sub.8, II.sub.10, V.sub.8 6 53 56 3 d II.sub.8, II.sub.10, IV.sub.9 8 41.5 47 5.5 e II.sub.8, III.sub.10, IV.sub.9 -30 46.8 51.0 3.2 f II.sub.8, II.sub.10, III.sub.10 -32.7 54.7 57.3 2.6 III.sub.12, IV.sub.9, VI.sub.8 ______________________________________
In Table 1, the respective abbreviations have the following meanings:
mp: Melting point (.degree.C.) PA1 SN: Smectic/nematic transition point (.degree.C.) PA1 NI: Nematic/isotropic transition point (.degree.C.) PA1 NR: Nematic range (.degree.C.) PA1 [=NI point (.degree.C.)--SN point (.degree.C.)]
In the column labelled "Compound" in the table, for example, I.sub.8 means the compound of n=8 of Formula (I), mentioned above.
With regard to the composition a, the nematic range is as extremely narrow as 0.5.degree. C., but the melting point is as high as 24.degree. C., which is in the vicinity of room temperature. Therefore, this composition is difficult for practical use, though it seems to be usable in experiments. On the other hand, with regard to the compositions e and f, the melting points are as low as -30.degree. C. and -32.7.degree. C., respectively, but the nematic ranges are as wide as 3.2.degree. C. and 2.6.degree. C., respectively. Thus these compositions are not always satisfactory.
The nematic range is a factor which has an influence on the writing rate and the display contrast of the display elements, and when the nematic range is wide, the writing rate is low and the contrast also is inferior. That is, when thermal writing is carried out within a wide nematic range, a long cooling time is required in the nematic phase in the step of forming an opaque smectic phase from an isotropic liquid via a nematic phase, which constitutes a factor of retarding the writing rate. Furthermore, when the nematic phase state remains for a long period of time, the liquid crystal is oriented by a substrate subjected to an orientation treatment (vertical orientation or homogeneous orientation), so that no scattered smectic A phase can be obtained, which constitutes a factor in deteriorating the contrast.
With regard to the melting point, if it is 0.degree. C. or lower, the compositions are considered to be suitable for typical uses. Because the thermal writing type liquid crystal device is used indoors, it can be considered that the device is usually not used in an environment of 0.degree. C. or lower.
In addition to the above-mentioned compositions, the other thermal writing liquid crystal compositions are disclosed in Japanese Patent Application Laid-open Publication Nos. 144383/1985, 252686/1985, 268790/1986, etc. but compositions having a nematic range of 2.degree. C. or lower and at the same time a melting point of 0.degree. C. or lower have not yet been found.
The object of the present invention is to provide a liquid crystal composition for thermal writing liquid crystal device elements, and a liquid crystal display element using the composition, and more specifically, a liquid crystal composition having characteristics of an extremely narrow nematic range and a low-melting phase transition point, and a thermal writing liquid crystal device element using the composition.